1. Field of the Invention
The present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method, and a computer program for performing mutual communication among a plurality of wireless stations like a wireless local area network (LAN). In particular, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program in which each communication station performs random access on the basis of carrier detection in accordance with the carrier sense multiple access with collision avoidance (CSMA) system.
To be more precise, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program for realizing random access in a communication environment in which a plurality of communication modes each having a transmission rate different from each other is intermixed. In particular, the present invention relates to a wireless communication system, a wireless communication apparatus, a wireless communication method and a computer program for realizing random access with a smaller overhead under a communication environment in which a plurality of communication modes each having a transmission rate different from each other is intermixed.
2. Description of the Related Art
By setting up a LAN by connecting a plurality of computers to each other, the sharing of information such as a file and data, and the sharing of peripheral equipment such as a printer can be achieved, and further the exchange of information such as the transfer of electronic mail, data, contents and the like can be preformed.
Conventionally, a wired LAN connection using an optical fiber, a coaxial cable or a twisted-pair cable has been generally used. In this case, line construction work is needed, and it is difficult to set up a network easily. Furthermore, the laying of a cable is troublesome. In addition, after setting up a LAN, because the moving range of an apparatus is limited by the length of a cable, the wired LAN is inconvenient.
Accordingly, a wireless LAN is noticed as a system for releasing a user from LAN wiring of the wired system. Because almost all of wiring cables can be omitted in a work space such as an office in case of the wireless LAN, communication terminals such as personal computers (PC's) can be relatively easily moved.
In recent years, as the wireless LAN system has become high in speed and low in cost, the demand of the wireless LAN has been remarkably increased. In particularly, in the most recent days, for performing information communication among a plurality of electronic apparatus existing around a person by setting up a small-scale wireless network among them, the introduction of a personal area network (PAN) has been examined. For example, different wireless communication systems using frequency bands such as a 2.4 GHz band and a 5 GHz band which are not required to be licensed by the competent authorities to use have been defined.
As normal standards with regard to the wireless network, Institute of Electrical and Electronics Engineers (IEEE) 802.11 (see, for example, Non-Patent Document 1), High Performance Wireless Local Area Network (HIPERLAN)/2 (see, for example, Non-Patent Document 2 or Non-Patent Document 3), IEEE 802.15.3, Bluetooth communication and the like can be cited. The IEEE 802.11 standard includes various wireless communication systems such as an IEEE 802.11a standard and an IEEE 802.11b standard according to the differences of a wireless communication system, a frequency band to be used, and the like.
A method of providing an apparatus to be a control station called as an “access point” or a “coordinator” in an area to form a network under the generalized control by the control station for constituting a local area network by means of a wireless technique is generally used.
A wireless network locating an access point therein widely adopts an access control method based on a band reservation, in which when a certain communication apparatus performs an information transmission, the communication apparatus first reserves a band necessary for the information transmission at an access point for using a transmission path in order not to generate any collisions with the information transmission of another communication apparatus. That is, the wireless network performs a synchronized wireless communication in which each communication apparatus in the wireless network is synchronized with each other by locating the access point.
However, there is a problem in which the usability of a transmission path is reduced to half when an asynchronous communication is performed between communication apparatus on a transmission side and a reception side in a wireless communication system locating an access point therein because the wireless communication through the access point is certainly necessary.
On the other hand, as an another method for constituting a wireless network, an “ad-hoc communication” in which terminals are directly perform wireless communications with each other asynchronously has been devised. In particular, in a small-scale wireless network composed of a relatively few clients positioned near to each other, the ad-hoc communication, by which arbitrary terminals can directly perform asynchronous wireless communications with each other without using a specific access point, is considered to be suitable.
Because there is no central control station in an ad-hoc type wireless communication system, the system is suitable for constituting, for example, a home network composed of household electric apparatus. An ad-hoc network has the following features. That is, even if a terminal is in trouble or the power source thereof is off, a routing can be automatically changed, and consequently the network is difficult to break. Also, data can be transmitted relatively long distance while keeping a high-speed data rate by making a packet hop a plurality of times between mobile stations. Many development examples with regard to the ad-hoc system are known (see, for example, Non-Patent Document 4).
For example, in an IEEE 802.11 series wireless LAN system, an ad-hoc mode in which terminals operate in an autonomous distributed way in peer to peer without locating any control station is prepared.
Hereupon, it is necessary to avoid contention when a plurality of users accesses the same channel. As a typical communication procedure for avoiding the contention, carrier sense multiple access with collision avoidance (CSMA) is known. The CSMA indicates a connection method of performing multiple access on the basis of carrier detection. Because it is difficult to receive a signal which a terminal itself has performed an information transmission thereof in a wireless communication, a terminal starts own information transmission after confirming the nonexistence of information transmissions of the other communication apparatus not by a CSMA/collision detection (CD) method but by a CSMA/collision avoidance (CA) method for avoiding any collisions.
A communication method based on the CSMA/CA is described with reference to FIG. 11. In the example shown in the drawing, it is supposed that there are four communication stations #0 to #3 under a certain communication environment.
Each communication station having transmission data monitors a medium state for a predetermined inter frame space, or a distributed coordination function (DCF) inter frame space (DIFS), from the last detection of a packet. When any media are clear, namely when there are no transmission signals, the communication station performs random backoff. Furthermore, when there are no transmission signals also in this period, a transmission right is given to the communication station.
In the shown example, after monitoring the medium state for an inter frame space DIFS, the communication station #0, which has the random backoff set to be shorter than that of the other peripheral stations, acquires the transmission right to be able to start a data transmission to the communication station #1.
At the data transmission, the communication station #0, or the transmission source, stores the information for a network allocation vector (NAV), and describes a period of time until the completion of the transaction of a data communication in a duration field of the header of a MAC frame (MAC header).
The communication station #1, or the transmission destination of the data frame, performs a reception operation of the data addressed to the local station for the duration of the Duration described in the MAC header. When the data reception has been completed, the communication station #1 returns an ACK packet to the communication station #0, or the data transmission source.
Moreover, the communication stations #2 and #3, which have received the data frame, and which are not the data transmission destinations, decode the description in the Duration field of the MAC header, and recognize the state in which the medium is occupied without monitoring the medium until the transaction ends to stop the transmission. The work is called that the peripheral stations “raise a NAV”, or the like. The NAV is effective over the duration indicated in the Duration field. For example, the duration until the communication station #1, or the reception destination, will return the ACK packet is specified as the Duration.
In such a way, according to the CSMA/CA system, contention is avoided while a single communication station acquires a transmission right, and while peripheral stations stop their data transmission operations during the duration of the data communication operation, and thereby collisions can be avoided.
Hereupon, it is known that a concealed terminal problem is generated in a wireless LAN network in an ad hoc environment. The concealed terminal indicates a communication station which a communication station on one side of a communication party can hear but a communication station on the other side of the communication party cannot hear in case of performing a communication between certain specific communication stations. Because no negotiations can be performed between concealed terminals, there is the possibility that transmission operations collide with each other only by the above-mentioned CSMA/CA system.
A CSMA/CA in accordance with an RTS/CTS procedure is known as a methodology for solving the concealed terminal problem. Also in the IEEE 802.11, the methodology is adopted.
In an RTS/CTS system, a data transmission source communication station transmits a transmission request packet Request To Send (RTS), and starts a data transmission in response to the reception of a confirmation note packet Clear To Send (CTS) from a data transmission destination communication station. Then, when a concealed terminal receives at least one of the RTS and the CTS, the concealed terminal sets a transmission stop duration of the local station for the duration in which the data transmission based on the RTS/CTS procedure is expected to be performed, and thereby collisions can be avoided. The concealed terminal for a transmission station receives the CTS to set a transmission stop duration for avoiding the collision with a data packet. The concealed terminal for a reception station receives the RTS to stop the transmission duration for avoiding the collision with the ACK.
FIG. 12 shows an operation example of the RTS/CTS procedure. Incidentally, it is supposed that there are four communication stations #0 to #3 in the communication environment of the wireless communication environment. The communication stations #0 to #3 are supposed to be in the following state. That is, the communication station #2 can communicate with the adjacent communication station #0. The communication station #0 can communicate with the adjacent communication stations #1 and #2. The communication station #1 can communicate with the adjacent communication stations #0 and #3. The communication station #3 can communicate with the adjacent communication station #1. However, the communication station #2 is a concealed terminal for the communication station #1, and the communication station #3 is a concealed terminal for the communication station #0.
Each communication station having transmission data monitors a medium state for a predetermined inter frame space DIFS (DCF Inter Frame Space) until the communication station has detected a packet last. When the medium is clear, namely when the there are no transmission signals, during this period of time, the communication station performs random backoff. Moreover, when there are no transmission signals also during this period of time, the communication station is given a transmission right.
In the example shown in the drawing, the communication station #0, which has set the random backoff shorter than that of the other peripheral stations after the monitoring of the medium state for the inter frame space DIFS, can acquire the transmission right to start the data transmission to the communication station #1.
That is, the communication station #0, which transmits data, transmits a transmission request packet (RTS) to the communication station #1. On the other hand, the communication station #1 being the reception destination returns a confirmation note (CTS) to the communication station #0 after a shorter inter frame space Short IFS (SIFS). Then, the communication station #0 responds to the reception of the CTS packet to start the transmission of a data packet after the inter frame space SIFS. Moreover, when the communication station #1 completes the reception of the data packet, the communication station #1 returns an ACK packet with an inter frame space SIFS put between. Because the inter frame space SIFS is shorter than the inter frame space DIFS, the communication station #1 can transmit the CTS packet before the other stations, which acquires the transmission right after waiting for DIFS+random backoff in accordance with a CMSA/CA procedure.
At this time, the communication station #2 and the communication station #3, both located at positions where both of them can be concealed terminals from both of the communication station #0 and the communication station #1, performs control to detect the use of a transmission path by the reception of the RTS or the CTS, and not to perform any transmissions until the communication ends.
To put it more specific, the communication station #2 detects the start of the data transmission of the communication station #1 as the transmission source on the basis of an RTS packet, and decodes the Duration field described in the MAC header of the RTS packet, and further recognizes that the transmission path has been already used after that for the duration until the successive transmission of the data packet is completed (the duration until the end of ACK). Thereby, the communication station #2 can raise a NAV.
Moreover, the communication station #3 detects the 25 start of the data transmission of the communication station #1 as the reception destination on the basis of the CTS packet, and decodes the Duration field described in the MAC header of the CTS packet, and further recognizes that the transmission path has been already used after that during the duration until the transmission of the successive data packet is completed (the duration until the ACK had ended). Thereby, the communication station #3 can raise a NAV.
In such a way, when a concealed terminal receives at least one of the RTS and the CTS, the concealed terminal sets the transmission stop duration of the local station for the duration to be expected to perform the data transmission based on the RTS/CTS procedure. Consequently, collisions can be avoided.
Now, the standardization of the IEEE 802.11g for supporting higher speed communication rate as a higher rank standard of the IEEE 802.11b being a wireless LAN specification using 2.4 GHz band has been advanced. A communication station in accordance with the IEEE 802.11g (hereinafter also referred to “high-grade communication station” simply) can also operate in accordance with the IEEE 802.11b, and can transmit a data packet also at a high-speed rate at which a conventional communication station in accordance with the IEEE 802.11b (hereinafter also referred to as “conventional station” simply) cannot perform any reception.
Hereupon, there is a problem of the coexistence of different communication systems, or a problem of the coexistence of the IEEE 802.11g and the IEEE 802.11b, both using the same band. That is, because the conventional station cannot receive a data packet to be transmitted at a high-speed rate, the conventional station cannot decode the Duration described in the MAC header, and cannot raise a NAV appropriately. Consequently, the conventional station cannot avoid collisions.
For example, in the example shown in FIG. 11, the communication station #0 and the communication station #1, both being communication parties, can exchange a data packet at a high-speed rate in conformity with IEEE 802.11g. On the other hand, when the communication station #2 and the communication station #3 around the communication station #0 and the communication station #1 are conventional stations which do not conform to the IEEE 802.11g, the communication stations #2 and #3 cannot decode the Duration described in the MAC header as a result of being unable to receive the data packet. Consequently, there is the possibility that the communication stations #2 and #3 start their communication operation even in the duration of the Duration to generate a collision (sec FIG. 13).
The present inventors consider that the problem of the coexistence of the IEEE 802.11g and the IEEE 802.11b is preferably solved by the setting of the IEEE 802.11g, being a higher rank standard, to assure ad-hoc compatibility.
For example, a method of performing the exchange of an RTS/CTS packet at a transmission rate at which a conventional station can receive the RTS/CTS packet before the transmission of a data packet in IEEE 802.11g can be considered (see FIG. 14). In this case, peripheral conventional stations decodes the Duration field described in the MAC header of the RTS/CTS packet, and recognize that the transmission path has already used for the duration until the completion of the transmission of the successive data packet after that (the duration until ACK ends). Thereby, the peripheral conventional stations can raise an NAV only for suitable duration. That is, the conventional stations cannot hear a data packet to be transmitted at a high-speed rate, but that turns to be no problem for avoiding a collision.
A procedure for securing a band in accordance with the above-mentioned procedure before the transmission of a data packet is generally called a virtual carrier sense.
However, in such a band securing procedure, the transmission of a data packet cannot be performed without performing the RTS/CTS procedure certainly not only in the case where the concealed terminal problem is generated, but also in the case where the concealed terminal problem does not exist. That is, the faster the transmission rate becomes, the larger the problem of an RTS/CTS overhead becomes. Also, the communication efficiency decreases by the degree of the problem.    Non-Patent Document 1: International Standard ISO/IEC 8802-11: 1999(E) ANSI/IEEE Std. 802.11, 1999 Edition, Part 11: Wireless LAN Medium Access Control (MAC) and PHYsical Layer (PHY) Specifications.
Non-Patent Document 2: ETSI Standard ETSI TS 101 761-1 Vl. 3.1 Broadband Wireless Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 1: Basic Data Transport Functions.    Non-Patent Document 3: ETSI TS 101 761-2 Vl. 3.1 Broadband Wireless Access Networks (BRAN); HIPERLAN Type 2; Data Link Control (DLC) Layer; Part 2: Wireless Link Control (RLC) sublayer.    Non-Patent Document 4: C. K. Tho, “Ad-Hoc Mobile Wireless Network” (Prentice Hall PTR Corp.).